Ž .Applied Surface Science 142 1999 516–520

Field emission characteristics of boron-doped diamond filmsprepared by MPE-CVD

Yung-Hsin Chen, Chen-Ti Hu, I-Nan Lin )

Department of Materials Science and Engineering, Materials Science Center, National Tsing-Hua UniÕersity, Hsin-chu, 300 Taiwan, ROC

Abstract

The effect of post-treatment process on electron field emission properties of the chemical vapor deposited diamond films,which were incorporated with boron-species, was examined. The SIMS profile and IR spectroscopy reveal that the solubility

Ž 3q. 21 y3limit of boron species in diamond materials is around B s5=10 cm , whereas the concentration of boron-species,Ž 3q. 20which can be incorporated as substitutional dopants in diamond lattice is one tenth of solubility limit, i.e., B s5=10

cmy3. The electron field emission properties of the as-deposited diamond films vary with the boron-content significantly.Moreover, post-annealing in vacuum alters the electron field emission properties of the films mainly via the rearrangementof the defects. Therefore, the electron emission capacity of lightly boron-doped diamonds is markedly suppressed and that ofthe heavily boron-doped diamonds is insignificantly modified due to post-annealing process. q 1999 Elsevier Science B.V.All rights reserved.

PACS: 79.70.qq; 73.90.q f

Keywords: Field emission; CVD diamonds

1. Introduction

Ž .Field emission devices FED with high emissionw xcurrent density have been attained in metal tip 1

w xand silicon tip arrays 2 . They have great potentialfor applications as electron emitters in flat paneldisplays and have attracted thorough investigations.However, the electrical field needed for triggeringthe field emission of these devices is rather high andtheir performance degrades rapidly due to thermaleffects, which results in serious contamination of,and damage to, the emission material.

) Corresponding author. Tel.: q886-3-5742574; Fax: q886-3-5716977; E-mail: inlin@msc.nthu.edu.tw

Diamond films possess negative electron affinityŽ . w xNEA characteristics 3 , in addition to the mar-velous properties such as high electron mobility,high thermal conductivity and chemical inertness.Therefore, these materials are considered to be highlypromising for applications in electron field emissiondevices and the related emission properties have

w xbeen widely investigated 4–11 . Converting dia-mond materials into p-type semiconductors byboron-doping has been observed to markedly en-

w xhance the electron field emission properties 12,13 .Moreover, the introduction of defects by depositionparameter control in the chemical vapor depositionŽ .CVD process has been reported to improve theelectron field emission properties of the materialsw x13–15 .

0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0169-4332 98 00687-4

( )Y.-H. Chen et al.rApplied Surface Science 142 1999 516–520 517

In this paper, we systematically examine the ef-fect of boron-doping and post-annealing on the elec-tron field emission behavior of the CVD-diamond. Apossible mechanism describing the effect is dis-cussed.

2. Experimental

Diamond films were grown on silicon substratesby a microwave plasma enhanced chemical vapor

Ž .deposition MPECVD method, using a ASTeX 5400Ž .system. The substrates, 100 p-type silicon with

resistivity 10 V cm, were cleaned by acetone anddeionized water prior to deposition. The diamondgrains were directly nucleated on mirror smoothsilicon surface using a y170 V bias voltage for 15

Ž .min, and then deposited with bias voltage q100 Vapplied in situ. In addition to the CH and H4 2Ž .CH rH s18r300 sccm gases used, boron-species4 2

were incorporated into the diamond films by bub-Ž . Ž .bling the H -gas 0–9 sccm into B CH O liquid2 3 3

precursors maintained at 108C. The total pressureand microwave power were controlled at 70 Torr and2500 W, respectively. The substrate temperature wasmaintain at around 9008C. The diamond films arearound 1.5 mm. Post-annealing was proceeded at9008C in vacuum for 45 min.

The depth profile of boron species of the CVDdiamonds is examined using secondary ion mass

Ž .spectroscopy SIMS . The boron concentration iscalculated from the ion counts in SIMS spectra usingthe experimental formula,

r s I rI PRSF 1Ž . Ž .i i m

Ž 3.where r is the atomic density atomsrcm foriŽ .dopant, I is the secondary ion counts countsrs fori

Ž .dopant, I is the secondary ion counts countsrsm

for matrix and RSF is the relative sensitivity factorŽof dopant-to-matrix ion concentration RSFs3=

21 y3 . w x10 cm for boron in diamond 16 .The morphology and the structure of the diamond

films were examined using scanning electron mi-Ž .croscopy SEM, Hitachi S-4000 , Raman spec-Ž .troscopy He–Ne Laser, Renishaw and infrared

Ž .spectroscopy IR, Bomem DA8 FT spectrometer ,respectively. The electron field emission behavior ofthe films was characterized by a diode setup, in

which the films were separated from the anode,Ž .In Sn O -coated glass, by using 50 mm glass1yx x 2

fibers as spacers. The current–voltage characteristicsŽwere measured by using an electrometer Keithley

.237 . The electron field emission properties werew xanalyzed using the Fowler–Nordheim model 17 .

The turn-on voltage was estimated as the voltage, atŽ 2 .which the log IrV y1rV curve deviated from

Fowler–Nordheim plot and the effective work func-Ž .tion F sFrb of the films were calculated frome

the slope of the Fowler–Nordheim plot, where F isthe field enhancement factor and b is the true workfunction of the materials.

3. Results and discussion

The electron field emission properties of the dia-mond films vary significantly with the proportion of

Ž .boron species incorporated. Fig. 1a solid circlesŽ .indicates that lightly doped samples B can be2

Ž .turned-on at relatively low field E s7.8 Vrmm0

and the emission current density attains J s58e

mArcm2, at 21.6 Vrmm applied field. By contrast,Ž .the heavily doped samples B need larger turn-on4

Ž .field E s10.2 Vrmm and show relatively small0Ž 2 .electron emission capacity J s27.6 mArcm ,e

shown as solid diamonds in Fig. 1c. The high B-con-tent samples possess markedly larger effective work

Ž .function F than the low B-content ones, viz.eŽ . Ž .F s0.066 eV and F s0.079 eV. Thesee B2 e B4

results infer that only when proper amount of p-typedopants are incorporated, the electron conduction indiamond crystals is improved and the emission ofelectrons is enhanced.

Post-annealing process markedly suppresses theelectron field emission capacity of the lightly boron-

Ž .doped diamond films B , open circles, Fig. 1a , but2

imposes insignificant effect on electron field emis-sion properties of the heavily boron-doped diamond

Ž .films B , open diamonds, Fig. 1c . The effective4

work function is essentially unchanged but the turn-on field increases pronouncedly. The samples con-

Ž .taining B OCH larger than 4 sccm behave in a3 3

similar manner with the B samples, indicating that4

incorporation of boron-species exceeding certain limitdegrades the electron field emission properties of the

( )Y.-H. Chen et al.rApplied Surface Science 142 1999 516–520518

Ž .Fig. 1. The electron field emission properties J –V curve of theŽ .as-deposited closed symbols and post-annealed in vacuum at

Ž .9008C for 45 min open symbols CVD grown diamond filmsŽ . Ž . Ž .doped with boron, where B OCH is a 2 sccm, b 3 sccm and3 3

Ž .c 4 sccm.

diamond films markedly. Moreover, it is interestingto observe that the electron field emission capacityof B samples increases significantly due to post-an-3

nealing treatment, with the effective work functionŽ .remaining roughly the same Fig. 1b .

The depth profile of boron species of the CVDdiamonds is examined using secondary ion mass

Ž .spectroscopy SIMS . Fig. 2 indicates that the largestconcentration of B-ions can be incorporated into thediamonds in CVD process is around 5=1021 cmy3

Ž . Ž .for B OCH s 6 sccm B samples. Higher3 3 6Ž .B OCH concentration in CH rH plasma than3 3 4 2

this value does not increase the boron-content in thediamond films. All of the CVD diamonds containfaceted grains of the size around 0.5 mm. Twinningoccurs almost to every grains, regardless of theŽ .B OCH concentration incorporated in the3 3

CH rH plasma, as shown in Fig. 3. The post-an-4 2

nealing process does not modify the microstructure

Ž .of the diamond films. When 2 sccm of B OCH is3 3

incorporated in the plasma, the diamond lattice ismarkedly distorted such that the D-band diamond

Ž y1 .peak 1332 cm is not only shifted but alsobroaden, inferring the presence of strains, shown asRaman spectra in Fig. 4a. In the extreme case, a verydiffused D-band resonance peak is observed for sam-

Ž .ples deposited with B OCH G4 sccm in CH rH3 3 4 2

plasma. Moreover, a broad DU-band occurs near1220 cmy1 for all CVD diamonds doped with B-species, which indicates, again, that these diamondsconsist of highly distorted lattices and contain largeproportion of atomic defects. The post-annealingprocess reduces the diffuseness of the D-band reso-nance peak of B and B samples markedly, which2 3

implies the healing of the atomic defects. But thestructure of the diamonds remains essentially un-

Ž .changed for the heavily doped B diamonds.4

An absorption peak corresponding to B–C bondis clearly observed in the vicinity of 1280 cmy1 of

Ž .infrared IR absorption spectroscopy for all the films

Fig. 2. The variation of boron concentration in diamond films withŽ .the proportion of B OCH in CH rH plasma.3 3 4 2

( )Y.-H. Chen et al.rApplied Surface Science 142 1999 516–520 519

Fig. 3. The SEM microstructure of diamond films grown by CVDŽ .process, containing B OCH in CH rH plasma.3 3 4 2

Ž .Fig. 4b , indicating that the boron species have beenincorporated as p-type dopants. The concentrationfor incorporating boron species as p-type dopants is

Ž 3q. 20 y3limited to around B s5=10 cm , whichS

Ž Ž . .corresponds to B samples B OCH s2 sccm .2 3 3Ž 3q.Higher B concentration than this value will in-

duce large proportion of atomic defects and result inthe formation of precipitates or inclusions, which actas electron traps. The presence of the atomic defectsis supported by the occurrence of large absorptionsignal in large wave number regime for B and B3 4

samples, since the atomic defects are very absorptiveto the infrared radiation. The resonance peak corre-sponding to B–C bond is not modified at all by thepost-annealing process, but a small resonance peakcorresponding to Si–C is induced in this process.

w xAccording to a previous report 18 , the impurityband is always induced in CVD diamonds and thepresence of impurity band has been proposed to bethe prime mechanism, resulting in low effective workfunction of these materials. The above describedresults imply that electron traps are induced whenthe boron concentration exceeds substitutional limitŽŽ 3q. .B such that the electron conduction in dia-S

monds is hindered. The boron species, which induce

Ž . Ž .Fig. 4. The a Raman spectroscopy and b infrared absorptionŽ . Ž .spectra of CVD grown diamond films, where B OCH is a 23 3

Ž . Ž .sccm, b 3 sccm and c 4 sccm.

( )Y.-H. Chen et al.rApplied Surface Science 142 1999 516–520520

Žthe atomic defects i.e., those which are not incorpo-.rated as substitutional dopants , readjust themselves

to reduce the induced strain in the post-annealingprocess. By contrast, the boron species, which areincorporated substitutionally as p-type dopants willnot change their site of occupancy such that theconcentration of acceptors remain the same for thepost-annealed diamond films.

The defect structure of the diamond films ismarkedly modified due to post-annealing process.The post-annealing process heals the atomic defectin B samples, which converts the deep electron3

traps into impurity levels and facilitates the jump ofelectrons from valence band toward conduction bandŽ .or surface states . The electron field emission capac-ity is thus significantly increased. The electron trap

Ž .levels existing in heavily doped materials B are4

too deep and the proportion of the electron traps aretoo abundant to be healed. Therefore, the post-an-nealing process can only slightly reduce the latticedistortion without effectively increasing the electronfield emission capacity. By contrast, the post-anneal-ing reduces the proportion of impurity levels for

Ž .lightly doped B samples, which markedly hinders2

the efficiency of electron transport from valenceband to the conduction band and significantly sup-presses their electron emission capacity.

4. Conclusion

The CVD diamond films incorporated with boronŽ .species were annealed in vacuum and their electron

field emission properties were examined. The post-annealing process modifies the thin films’ propertiesmainly through the rearrangement of the defects.This process heals the atomic defects for lightlyboron-doped diamond films and converts the deepelectron traps into impurity levels for the heavilyboron-doped films. The electron field emission prop-erties of these films are thus significantly changed.

Acknowledgements

The authors would like to thank the NationalScience Council, Republic of China, for the financialsupport through the project NSC 88-2112-M-007-006.

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